To determine the effect of arsenic on the immune response to Dukoral, we assume that the immunogenicity of the vaccine among children in the arsenic non-contaminated area will be 55% and in the contaminated area it will be reduced to 35%. To detect this difference with 95% confidence and 90% power, we estimated a sample size of 138 in each of the two study areas. With around 10% loss to follow up the required sample size will be 154. Thus, a total of 308 (154 x 2 = 308) children, aged 2-5 years, will be enrolled.